Field of the Invention
The present invention relates to the industrial art of earth-boring and well drilling for the recovery of fluid minerals. More particularly, the invention relates to a carrier for a multiplicity of shaped explosive charges to penetrate well casing with multiple apertures.
Description of Related Art
In the oil and gas industry, well plugging operations are often performed to seal wellbores in order to abandon the wells. Eventually, all wells exhaust their purpose and are abandoned. Either the well is a “dry hole”, having no economically viable production, or has depleted the production strata. In either case, a non-productive well is or should be permanently “plugged”. “Plug and abandonment” procedures are required under various state and federal laws and regulations. Plug and abandonment operations performed upon a cased wellbore require that at least a section of the wellbore be filled with cement to prevent the upward movement of fluids toward the surface of the well. To seal the wellbore, a bridge plug is typically placed at a predetermined depth in the wellbore and thereafter, cement is injected into the wellbore to form a column of cement high enough to ensure the wellbore is permanently plugged.
In addition to simply sealing the interior of a wellbore, plug and abandonment regulations additionally require that an area outside of the wellbore be sufficiently blocked to prevent any fluids from migrating towards the surface of the well along the outside of the casing. Migration of fluid outside of the casing is more likely to arise after a fluid path inside the wellbore has been blocked. Additionally, where multiple strings of casing line a wellbore, the annular area between concentric casing strings can form a fluid path in spite of being cemented into place when the well was completed. Inadequate cement jobs and deterioration of cement over time can lead to flow paths being opened through an otherwise solid cement barrier.
There are several reasons to line a well borehole with two or more substantially concentric casings. As one example, two or more mineral strata may be produced from the same borehole. In this example, a smaller diameter casing is set within a larger diameter casing. A first mineral stratum of oil, gas or both, may be produced along the flow annulus between the two casings. A second, usually deeper mineral stratum is produced along the flow bore of the smaller or innermost casing. This sequence may be repeated for multiple pay strata and multiple concentric casings.
Another example of multiple concentric casings is that of extremely deep borings that require a tapered casing string to line an unstable raw borehole along a greater depth than normally expected of a surface casing. In this context, a “tapered” casing string means one in which an inner casing of smaller OD than the ID of an outer casing is secured to the end of the outer casing. Although the surface casing may not penetrate a mineral bearing stratum, the annulus between two concentric casings may carry a flow of gas that has escaped an inner flow bore.
Many off-shore, deep water wells have extremely large surface casings; in the order of 24″ ID. These large surface casings are set to a bottom hole depth of 3,000′ to 5,000′ below the seafloor. The seafloor may be under an ocean depth of 1,000′ to 5,000′ below a drilling rig floor.
When a well is abandoned, all of the productive flow channels must be filled with cement to a designated depth below the surface or seafloor. In the case of multiple casings, there are two possible approaches available for sealing all of the annuli present. In one approach, as represented by U.S. Pat. No. 5,472,052 to P. F. Head, all of the upper ends of casings that are interior of the outermost casing are milled away down to the designated depth. Thereafter, a solid core of cement is placed to fill the interior volume of the outermost casing. The annulus between the outermost casing OD and the raw borehole ID is filled with cement when originally set.
An alternative well plugging procedure is to set a bridge plug within the innermost casing and perforate the inner casing wall above the plug. Cement is pumped down the inner casing and forced out into the annulus between the inner and outer casings. For multiple annuli, this process is repeated by the selective use of shaped charges that will perforate only the desired number of casing walls but not the outermost casing.
Of the two procedures available for plugging an abandoned well, the latter procedure of casing wall perforation and filling the one or more annuli with cement is more economical by several orders of magnitude. However, deep water offshore wells present unique difficulties for this alternative procedure. When originally drilled, a large drilling platform or drill ship was used to support the immense weights and forces necessary to drill such wells. A “riser” of greater diameter than the largest casing to be set in a particular well links the surface casing to the drilling rig to protect the borehole from invading seawater and as a conduit for the return flow of drilling fluid. When the drilling and well preparation is complete the drilling platform is removed along with the large riser. Smaller and lighter drill ships capable of supporting considerably smaller risers, in the order of 6⅝″, are used for well maintenance. By the time of well abandonment, platforms such as was used for the original drilling, are not economically available. In many deep water wells, however, even the smallest or innermost casing is larger than the riser capacity of most maintenance ships.
Casing perforations utilized in a cement “squeezing” operation are typically formed with a perforating assembly that includes a number of shaped charges. An apparatus representative of this concept includes resiliently biased members that remain in contact with the casing wall as the apparatus is lowered into the well. The shaped charges are mounted on the inside surface of bars that are resiliently biased to maintain physical contact with the interior casing wall. The shaped charges are secured at a predetermined distance from the inside bar surface as determined by the casing wall thickness and/or the number of casing walls to be penetrated. An example of such a resiliently biased perforating gun is disclosed in U.S. Pat. No. 5,295,544 to D. V. Umphries. However, the radial expansion distance of a prior art resilient bar is insufficient to accommodate the radial difference between a 6⅝″ maintenance ship riser and a 24″ casing.